1. Field of the Invention
The present invention relates to rotary drive devices for driving joints, such as links and hands, of a robot arm, and, more specifically, it relates to rotary drive devices for rotationally driving joints via reduction gears.
2. Description of the Related Art
A typical conventional rotary drive device for driving a link or hand of a robot arm uses an AC or DC servo motor and is connected to the object to be driven, with a reduction gear provided on the output side of the motor to obtain a high-output torque. In recent years, robot arms are required to achieve higher precision, higher speed, and higher output. Japanese Patent Laid-Open No. 2006-141101 proposes a rotary drive device that uses a reduction gear, in which the position of the origin with respect to the rotation shaft of the reduction gear is detected. Furthermore, Japanese Patent Laid-Open No. 5-252779 discloses a control structure that controls the torque of a servo motor according to the output from a torque sensor attached to the output side of a reduction gear to suppress the vibration of the robot arm. Moreover, Japanese Patent Laid-Open No. 2003-18880 discloses a rotary drive device in which a rotational-speed detecting unit is provided on each of the rotation shaft of the motor and the rotation shaft of the reduction gear so as to bring the rotational speed of the rotation shaft of the reduction gear to a preset speed.
However, because the rotary drive device rotationally drives the object to be driven via the reduction gear, an inner mechanism of the reduction gear may be deformed or distorted, due to insufficient strength, by the torque acting on the rotation shaft of the reduction gear. In such a case, even if the rotation shaft of the motor is set to a target angle reference value, the rotation shaft of the reduction gear may be twisted in the rotation direction by the torque acting on the rotation shaft of the reduction gear. In the conventional rotary drive device, if the rotation shaft of the reduction gear is shifted from the ideal position due to twisting like this, high-precision positioning becomes difficult. Accordingly, it is difficult to improve the precision of movement, such as linear movement, of robot arms. Furthermore, vibration of the rotation shaft of the reduction gear cannot be suppressed because of insufficient strength of the reduction gear. This increases the settling time and, hence, the time before starting the operation of the robot arm.
More specifically, the provision of an origin sensor on the rotation shaft of the reduction gear, as in the above-described conventional rotary drive device, is not enough for high-precision positioning, because it simply detects the position of the origin and, thus, the resolution of position detection is insufficient. Furthermore, even if the torque sensor is provided on the rotation shaft of the reduction gear, as in the above-described conventional rotary drive device, it is impossible to detect the amount of deformation of the inner mechanism or the amount of shift of the rotation position of the rotation shaft, due to distortion, caused by the torque acting on the inner mechanism via the rotation shaft of the reduction gear. Therefore, it is impossible to position the rotation shaft of the reduction gear with high precision. In addition, detecting the rotational speed of the rotation shaft of the motor and the rotational speed of the rotation shaft of the reduction gear, as in the above-described conventional rotary drive device, does not mean detecting the torque acting on the rotation shaft of the reduction gear or detecting the rotation position of the rotation shaft of the reduction gear. Therefore, it is impossible to position the rotation shaft of the reduction gear with high precision.
Accordingly, the present invention provides a rotary drive device for a robot arm with an increased positioning precision of the rotation shaft of the reduction gear, which can suppress vibration of the robot arm by torque control and reduce the settling time.